In wellbore operations, it is sometimes necessary or desirable to enlarge a diameter of a wellbore section for fitting different pieces of equipment in downhole locations. Traditionally, enlargement of a wellbore has been carried out by performing an underreaming operation (after a well has been drilled) using an underreamer tool provided with cutting devices typically provided on extendable and retractable arms. Such a tool is fitted to a string of tubulars or jointed pipe which is then rotated to turn the underreamer so that it cuts into a section of the inner wall of the wellbore. For example, an underreamer may be run in an 8 inch (0.2032 meters) open hole section of the wellbore to expand its diameter to around 10 inches (0.254 meters). The section of wellbore wall may be lined with a tubular or casing in which case the operation is referred to as a milling operation which can be conducted with similar tools to an underreamer with suitable modifications to the cutting elements, or may be an open hole (non-lined) section exposed to the geological formation.
More recently, underreamers have been incorporated in the same string as used for a drilling operation, i.e. a drill string, to mitigate costs which would otherwise be required to complete a separate reaming run into the wellbore. Such underreamers may be designed to be positioned closely behind the drill bit itself, providing a “near bit” underreamer as known in the art.
Typically, the cutting devices of the underreamers are actuated when required. In order to do so, a mechanical actuation device can be employed to force the cutting devices radially outwards. However, these can suffer from problematic frictional effects of the interaction of the actuation components, and as the cutting elements come into contact with the wellbore wall, the forces encountered may urge the cutting elements back toward their non-actuated positions.
Hydraulic actuation devices are also known in such tools, where for example the cutting elements are movable outward radially into the wellbore annulus by applying pressure inside the tool acting directly on axially arranged pistons that drive cams, racks or levers, against the pressure of fluid circulating in the wellbore annulus. Such tools work on the basis that the pressure required inside the tool typically needs to overcome the pressure of fluid in the wellbore annulus, which may vary so that it may be difficult to predict at what point the tool is opening because there is no definite threshold of pressure differential required to be applied inside the tool to move the cutting devices. Additionally, the piston areas are geometrically constrained due to the nature of the space available in the wellbore and the resultant radial forces which may be applied to the rock face may be insufficient for the purposes of rock removal.